a) Field of the Invention
The invention is directed to a method and an arrangement for measuring the anterior segment of the eye. It is applicable in particular for determining the parameters needed for selecting a suitable intraocular lens for a cataract operation. Further, it can also be used for quality control subsequent to the implantation of an intraocular lens.
b) Description of the Related Art
A device that can be used for contactless measurement of the eye length, corneal curvature and anterior chamber depth is known from DE 198 57 001. In this connection, the axial length is determined interferometrically, the corneal curvature is determined by means of image processing from reflection images of measurement marks that are projected on the cornea at a determined angle, and the anterior chamber depth is determined from the evaluation of the back-scattering of a column-shaped illumination of the eye length.
The described measurement of the anterior chamber depth does not function in the presence of pseudophakia because the implanted intraocular lenses (IOL) generally do not have a scattering effect. The interferometric measurement of the axial eye length is known from “Optical Measurement of the Axial Eye Length by Laser Doppler Interferometry” (Ch. Hitzenberger, Investigative Opthalmology and Visual Science, Vol. 32, No. 3, page 616, March 1991), which disclosure is referred to in the following.
DE 101 08 797 describes a method for determining the diameter of the pupil and iris with digital image processing means, wherein the angle between the visual axis and optical axis of the eye, among others, can also be determined.
A test setup by which the anterior segment of the eye can be measured by interferometry is described in “Submicrometer Precision Biometry of the Anterior Segment of the Human Eye” (Drexler et al., Investigative Opthalmology & Visual Science, Vol. 38, No. 7, page 1304, June 1997). For this purpose, the eye is irradiated by a collimated light bundle during the measurement process. The light components which are reflected by the cornea and lens surfaces and are imaged on a photodetector are relatively weak. The eye must be oriented for measurement in such a way that its optical axis coincides with the measurement axis of the device. For this purpose, a collimated fixating light is presented to the patient along a stationary (coaxial) axis, which fixating light is coupled in by a mirror for the eye to be measured. The adjustment of an angle between the visual axis of the patient and the measurement axis of the test setup is carried out by means of a scanning mirror. Even with a deviation of the optical axis from the measurement axis in the range of 1° (e.g., due to fixating problems or nystagmus), the reflections of the cornea and lens can no longer overlap so that there is no interference measurement signal. Accordingly, the measurement is very sensitive to tilting of the patient's eye. Further, the fixating light always appears in infinity to the patient, which can be disadvantageous. The position of the optical axis is found by tilting the scanning mirror in two directions orthogonal to one another until all measurement signals of the cornea and lens are to be detected simultaneously. This method is extremely time-consuming and also does not lead to the desired results in all patients. This method is too complicated for routine clinical use.